Density Distribution and Shape of Galactic Dark Halo Can be Determined by Low Frequency Gravitational Waves ?

Abstract

Under the assumption that the Milky Way's dark halo consists of primordial black hole MACHOs (PBHMACHOs), the mass density of the halo can be measured by the low frequency gravitational waves ($10^{-3} Hz \siml ν_{gw} \siml 10^{-1}$ Hz) from PBHMACHO binaries whose fraction is $\sim 10^{-6}$. We find that ten years observation by LISA will detect $\sim 700$ PBHMACHO binaries and enable us to determine the power index of the density profile within 10% (20%) and the core radius within 25% (50%) in about 90% (99%) confidence level, respectively. The axial ratios of the halo may also be determined within $\sim 10%$. LISA and OMEGA may give us an unique observational method to determine the density profile and the shape of the dark halo to open a new field of observational astronomy.